(to appear in International Journal of Fracture; Proceedings of the 5th Intl. Symposium on Defect andMaterial Mechanics)

Amit Acharya and Claude Fressengeas

The duality between terminating discontinuities of fields and the incompatibilities of their gradients is used to define a coupled dynamics of the discontinuities of the elastic displacement field and its gradient. The theory goes beyond standard translational and rotational Volterra defects (dislocations and disclinations) by introducing and physically grounding the concept of generalized disclinations in solids without a fundamental rotational kinematic degree of freedom (e.g. directors). All considered incompatibilities have the geometric meaning of a density of lines carrying appropriate topological charge, and a conservation argument provides for natural physical laws for their dynamics. Thermodynamic guidance provides the driving forces conjugate to the kinematic objects characterizing the defect motions, as well as admissible constitutive relations for stress and couple stress. We show that even though 'higher-order' kinematic objects are involved in the specific free energy, couple stresses may not be required in the mechanical description in particular cases. The resulting models are capable of addressing the evolution of defect microstructures under stress with the intent of understanding dislocation plasticity in the presence of phase transformation and grain boundary dynamics.

Hi,

I would like to model a microstructure using Abaqus. My question is how to deal with the grain boundaries ? Should I use cohesive elements ? Any ideas on modeling grain boundaries using FEM will be very helpfull.

Thanks,

Ayoub

We study the effects of grain boundary adhesion and grain size on the ductility of thin metal films well bonded to polymer substrates, using finite element method. It is shown that the ductility of polymer-supported metal films increases approximately linearly as the grain boundary adhesion increases, and as the grain size decreases. A rule-of-thumb estimate of the ductility of polymer-supported metal films agrees well with the simulation results.

In press, Scripta Materialia, 2008 

DOI: 10.1016/j.scriptamat.2008.06.058

When dislocation meet the grain boundary. The grain boundary present obstacles to dislocation motion. A usual point is that the dislocation will pile up against the grain boundary(But from the expriments , it seldomly see this phenomenon.). Macroscopic yielding occours when the adjacent grain can deform plastically Which maybe effected by the emission of dislocation from the grain boundary, or in another way , the pileup dislocation can produce a stress concentration which can active the dislocation source in the adjacent grain. As we knowe ,in polycrystalline, the material will express a Hall-Petch relationship between the yielding stress and the grain dimension. There are many models to explain the H-P relationship,such as pile-up dislocations, average fress distance....